Crystal bridge circuit for radio amplifiers



E. TOTH Sept. 29, 41942.

CRYSTAL BRIDGE CIRCUIT FOR RADIO AMPLIFIERS Filed Jan. 15, 1941 GRID RETURN INVENTOR Emerz'ck 76% BY ATTOR Y Patented Sept. 29, 1942 OFFICE'2.29am CRYSTAL names CIRCUIT FOR RADIO AMPLIFIERS Emerick Toth,Cheverly, Md. Application January 15, 1941, Serial No. 374,439 4 Claims.(Cl. 173-44) (Granted under the act of March 3, 1883, as amendedApriiso', 1928; 370 o. G. 757) This invention relates toa crystal bridgecircuit for R. F. or I. F. amplifiers and has for its object to providea bridge'circuit which will eliminate the serious detuning normallycaused between the input and output transformers in I the usual crystalbridge circuits when the transformers are aligned, either with crystalsout and crystals subsequently switched into the circuit, or when thetransformers are aligned with crystals in and the crystals aresubsequently switched out of the circuit.

The single figure of the drawing matic view showing the circuitsinvolved.

In the drawing reference character Ti-indicates a crystal circuit inputR. F. or I. F. transformer. T2 is the crystal circuit output R. F. or I.F. transformer. L1 is the primary and L2 is the secondary of transformerT1, La and L4 being the primary and secondary respectively oftransformer T2. The bridge circuitcomprises two 1 branches, one of whichcontains crystal X'I'L1 and the other of which contains crystal XTLe.

The two crystals are usually 50 to 500 cycles apart in frequency inorder to give a band-pass filter effect to thecircuit. S1 is adouble-pole, double-throw switch which is provided for the purpose ofswitching the crystals either in or out of the circuit. Condensers C1and C2 are provided according to the invention in order to introduceinto the branches of the circuit the proper coupling capacities toprevent detuning is a diagramwhen the crystals are switched either in orout of the circuit following alignment of the transformers with thecircuit in the opposite condition.

At the resonant frequency of either of the two crystals the secondary ofT1 is fairly tightly coupled to the primary of T: if condensers C1 andC2 are not present, although 100% coupling will not be achieved. Tightcoupling at frequencies other than the crystal frequencies is preventedmainly by the capacity present in the crystal holders. With the switchS1 in the crystals out position the coupling between the twotransformers is approximately 100% if condensers C1 and C2 are omitted.If the two transformers are aligned with S1 in this position seriousmistuning .will occur if the switch is then shifted to the B or crystalsin position, due to the introduction of the crystal holder capacity andthe resonant crystals. Likewise, serious mistuning will occur if thealignment is accomplishedwith the switch on the B or crystals inposition and the crystals are subsequently switched out of the circuit.

I The addition of condensers C1 and Cz serves to provide the propercoupling capacity with the crystals -in the circuit, fora degree ofcoupling comparable to that utilized in the transformers to obtain thedesired band width with the crystals out of the circuit. It is thusimpossible to .exceed the degree of coupling between the secondary of T1and the primary of T2 determined by C1 and C2 and the variation incoupling caused by switching .from the A to B position is greatlyreduced.

In addition, the selectivity curves resulting from the entire circuitare much more symmetrical, since in the A position C1 and C2 provide theproper degree of coupling for operation without crystals and in the Bposition the proper coupling is obtained at the resonant frequency ofthe crystalawhile the cap'acityeflects of the crystal holders, arisingat frequencies ofi crystal resonance are minimized by the relativelysmall series capacities of C1 and C2.

The use of single condensers in the same man'- ner is applicable tosingle crystal operation. Typical values for. kc. operation are asfollows: Cl =C2 10 mmf., L1 L2, L3 and L4 1 millihenry.

While the disclosure has been restricted to a single embodimentwhichapplicant now considers to be the preferred form of the invention, thescope of the invention is to be considered as restricted only by thescope and limitations of the appended claims.

The invention described herein may be manufactured and/or used by or forthe Government of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

I claim:

1. A crystal bridge circuit comprising two branches, a plurality ofcoupled resonant elements in said circuit, a crystal in-each of saidbranches, said crystals being resonant at differ- I entfrequencies,'means for switching said crystals into or out of saidcircuit, anda capacitorin each of said branches between said crystal andthe input end of said circuit, said capacitors providing approximatelycritical coupling be.- L tween said elements in order to substantiallyprevent detuning of said circuitwhen said crysin series with saidcrystal and located between said crystal and the input end-oi saidcircuit, said capacitor providing approximately critical cou-,

plins between said elements in order to substancuit subsequent to itsalignment in the alternate condition.

3. A crystal bridge circuit comprising a crystal,

a pluralityioi coupled resonant elements in said" 10" circuit, means forswitching said crystal either,-

into or out 0! said circuit. and means introducing into said circuit acapacitance in series with said crystal of such value as to provideapproximately critical coupling between said elements to substantiallyprevent detuning of said circuit when said crystal is switched eitherinto or out of said circuit subsequent to its alignment in the alternatecondition.

4. A crystal bridge circuit comprising two branches, a plurality ofcoupled resonant eletlally prevent detuning 0! said circuit when said 5crystal is switched either into or out of said cirments in said circuit,a crystal in each of said branches, said crystals being resonant atdifferent frequencies, means for switching said crystals either into orout of saidcirouit, and

means'introducing into each of'said branches a capacitance in serieswith its crystal 01' such value as to provide approximately criticalcoupling between said elements to substantially prevent detuning 01'-said circuit when said crystals are switched either. into or out oi'said 'cireuit I01- EMERICK TOTH.

